That's funny! Just tell her already! Say "I loved that dress you wore the other day so much, I went and bought it for my cruise!" It's not a big deal at all, it's not like you're both wearing the same prom dress to prom.
This post does remind me of my best friend when I was in 5th-7th grade. I loved that girl so much, and everybody called us "twins" even though she was like 4 or 5" taller than me...we had similar loud laughs, freckles, the same scary sense of humor, and we liked the same clothes. Funnily, I'd like one boy, she'd either like the same boy (if it wasn't serious) or if I was really liking him, she'd like his brother. Except she loved Joey from New Kids on the Block and I loved Sebastian from Skid Row or Nikki Sixx from Motley Crue. We just laughed it off as pre-teen stuff. I moved away, kept in touch 2 or 3 years, but then lost touch and I didn't see or talk to her for a solid 10 years, and one day I was driving through the old neighborhood and saw her dad in the yard. I stopped and asked about my friend, and he gave me her number. We made plans to meet at her parents' house to go to "our" restaurant at the mall (where else?) and catch up. When I drove up in my dark blue car in my levis, black heeled boots, 1980s Tshirt, silver bracelet, ring, and earrings, and my hair back, I just burst out laughing to see her in her similar model, dark blue car, with very nearly the same outfit, hair pulled back, silver bracelet, rings and earrings, and we had both even gotten a little "fluffy"! We just laughed and laughed, her mom cried which made it more funny. At the mall, I told her I wanted her to go into this store to smell this awesome lotion I'm all about---she guessed what it was, because that's what she has at home too. Sometimes, it really is just natural to be similar. Our lives couldn't have been any more different, but we still definately shared similar taste in "stuff".